The Vapor-phase Axial Deposition (VAD) method of fabricating multimode optical fibers begins with the deposition of germanium phosphosilicate soot particles, which are formed by the hydrolysis of SiCl.sub.4, GeCl.sub.4 and POCl.sub.3 in an oxy-hydrogen flame, on the end of a rotating vertical bait rod. The bait rod is slowly withdrawn vertically, from a deposition chamber, resulting in a substantially cylindrical, porous, soot-form. The resulting soot-form is subsequently dehydrated and consolidated at an elevated temperature into a clear glass boule. The consolidated glass boule is then stretched into a long rod that is to be used as the core of a preform from which multimode lightguide fiber is drawn. The clad of the preform is formed by the so-called "rod-in-tube" method, which entails the collapse of a silica tube over the core rod which is inserted therein.
However, for single-mode optical fibers, as described in U.S. Pat. No. 4,345,928 to Kawachi et al., which is incorporated by reference herein, the cladding material cannot be fabricated solely by the rod-in-tube method. Because of the small diameter of the single mode fiber core (e.g., 5 to 10 .mu.m), part of the lightwave traveling therealong finds passage in a portion of the clad near the core. The rod-in-tube method of fabricating all the cladding material results in a fiber that has high attenuation due to the presence of OH in the cladding. Thus, for low loss single-mode fibers made by the VAD method, part of the cladding material must be fabricated, like the core, with a soot deposition step to be followed by dehydration before consolidation into a clear glass lightguide preform.
Typically, the amount of cladding material to be fabricated in this manner must give a soot-form that has a deposited core-to-clad diameter ratio of 3 to 6. Thus, for single-mode fiber fabrication, the VAD method entails the use of several oxy-hydrogen torches for simultaneous soot formation; one torch for the deposition of a core and one or more torches to deposit the clad. After dehydration and consolidation, the resulting clear glass boule or preform is also stretched and extra cladding material is added by the above-described rod-in-tube method to attain the proper clad-to-core diameter ratio to give the desired cutoff wavelength. Typically for a 125 .mu.m outside diameter fiber with a cutoff wavelength of 1.15 .mu.m and a step index of .DELTA.n=0.004, the core has a diameter of approximately 8 .mu.m.
In multimode VAD fabrication, a circular cross-section torch comprised of five concentric, spaced quartz tubes is used to make the soot-forms for fabricating the core of the preform. Two inner tubes deliver the chemicals, SiCl.sub.4, GeCl.sub.4 and POCl.sub.3, while the other three tubes supply H.sub.2, Ar and O.sub.2 for the oxy-hydrogen flame. Typically, a soot-form that is made with this torch configuration has a core diameter in order of 50 mm. If a similar torch configuration is employed for single-mode fiber fabrication, the overall soot-form which encompasses both the core and clad would be overwhelmingly large (e.g., 300 mm). Such large soot-forms have built-in stresses making handling and sintering extremely difficult. Thus, for single-mode fiber fabrication, a technique must be developed to fabricate the core of the single-mode soot-form having a diameter less than 25 mm.
The Kawachi et al. patent states that the use of a cylindrical torch, even with the smallest chemical delivery orifice or the most convergent flame can only make soot forms that have a core diameter greater than 30 mm. They have resorted to the use of torches that are either rectangular or oval in shape with the chemical delivery port placed eccentric to the center of symmetry of the torch. In such manner, they have reported the growth of soot forms with core diameters of 10 to 20 mm. Unfortunately, the fabrication of the rectangular quartz torches with an offset delivery port is time consuming and expensive.
Accordingly, there is a need for an alternative technique for the fabrication of soot-forms, having small diameter cores, using a cylindrical torch configuration.